Annihilation of exceptional points from different Dirac valleys in a 2D photonic system
Mateusz Król, I. Septembre, Przemysław Oliwa, Mateusz Kędziora, K. Łempicka-Mirek, Marcin Muszyński, Rafał Mazur, Przemysław Morawiak, Wiktor Piecek, Przemysław Kula, W. Bardyszewski, Pavlos G. Lagoudakis, D. D. Solnyshkov, G. Malpuech, Barbara Piętka, Jacek Szczytko
Abstract
Topological physics relies on Hamiltonian's eigenstate singularities carrying topological charges, such as Dirac points, and - in non-Hermitian systems - exceptional points (EPs), lines or surfaces. So far, the reported non-Hermitian topological transitions were related to the creation of a pair of EPs connected by a Fermi arc out of a single Dirac point by increasing non-Hermiticity. Such EPs can annihilate by reducing non-Hermiticity. Here, we demonstrate experimentally that an increase of non-Hermiticity can lead to the annihilation of EPs issued from different Dirac points (valleys). The studied platform is a liquid crystal microcavity with voltage-controlled birefringence and TE-TM photonic spin-orbit-coupling. Non-Hermiticity is provided by polarization-dependent losses. By increasing the non-Hermiticity degree, we control the position of the EPs. After the intervalley annihilation, the system becomes free of any band singularity. Our results open the field of non-Hermitian valley-physics and illustrate connections between Hermitian topology and non-Hermitian phase transitions.